The present invention relates to a charging and discharging control circuit for controlling charging and discharging of secondary batteries and to a charging type power supply device having secondary batteries with the charging and discharging control circuit built therein.
As a conventional charging type power supply device comprising a secondary battery, a power supply device shown in a circuit block diagram of FIG. 2 is known. Such a structure is disclosed in, for example, Japanese published unexamined patent application number H4-75430(1992) entitled "CHARGING TYPE POWER SUPPLY DEVICE." More specifically, a secondary battery 101 is connected to an external terminal -V0 or +V0 through a switch circuit 103. Further, a charging and discharging control circuit 110 is connected in parallel to the secondary battery 101. The charging and discharging control circuit 110 has a function of detecting the voltage of the secondary battery 101. In the case that the secondary battery 101 is in either in an over-charged state (a state where the battery voltage is above a predetermined value --hereinafter referred to as an over-charge protection state) or in an over-discharge state (a state where the battery voltage is below a predetermined value --hereinafter referred to as an over-discharge protection state), a signal is output from the charging and discharging control circuit 110 to turn off the switch circuit 103. Further, it is possible to restrict the current through the switch circuit 103 by making, when the external terminal +V0 reaches a certain voltage, the switch circuit 103 turn off to stop the discharging. In other words, the discharging can be stopped when the current is excessive (over-current control). This state is hereinafter referred to as an over-current protection state. It is the role of the charging and discharging control circuit to protect the battery against these states.
Also, in the case that a plurality of secondary batteries are serially connected, a power supply device having a charging and discharging control circuit for detecting over-charge and over-discharge of the respective batteries can be similarly realized.